5. RESULTS

In Figure 4 we show the frequency histograms of
the asymptotic FUV and
NUV AB magnitudes, FUV luminosity and (FUV-NUV) (both asymptotic and
at the D25 aperture) color.
Heckman et al. (2005)
have recently shown that galaxies with FUV luminosities brighter than
2 × 1010
L (7.6 ×
1036 W or MFUV = -19.87) (also known as
ultraviolet-luminous
galaxies or UVLGs) are extremely rare in our Local Universe. Their
comoving space density is only ~ 10-5 Mpc-3, i.e.
several hundred times lower than that of their z = 3
counterparts, the Lyman Break Galaxies (LBG). Indeed, only four galaxies
in the Atlas
(see Figure 4c) would be classified as UVLGs:
two AGN, NGC 7469 and
Mrk 501, and two actively star-forming interacting
systems, the Cartwheel (see e.g.
Amram et al. 1998)
and UGC 06697
(Gavazzi et al. 2001).

The color distribution of Figure 4d shows a
pronounced peak at
(FUV-NUV) 0.4 mag and a
long tail extending to very red
colors. As we will show later, this red tail is, not unexpectedly,
mostly populated by elliptical galaxies of intermediate mass that show
little recent star formation activity and a weak UV-upturn (see
Boselli et al. 2005).
This figure also shows the distribution of
effective radii both in arcsec (Figure 4e) and
in kiloparsecs
(Figure 4f). The distribution of effective radii
is very similar for
the FUV and the NUV. Due to the limited spatial resolution of the
GALEX data we only computed the effective radius of galaxies for which
the semi-major axis
of the ellipse including 50 per cent of the light was larger than
6arcsec in radius. This fact, along with the lower limit in optical
diameter (1 arcmin) imposed by the completeness of the RC3, results
in a paucity of compact galaxies and a relatively narrow distribution
in apparent effective radius peaking at ~ 15 arcsec. The
distribution in physical size (Figure 4f), on
the other hand, is significantly wider with a peak around 5-6 kpc.

The distributions of the concentration indices C31 and C42
(Figures 4h & 4i,
respectively) are also very narrow with the galaxies being
slightly more concentrated (i.e. larger values of C31 and C42) in
the NUV than in the FUV (see Figures 4j &
4k for a comparison
between the value of these indices in the two bands). This is probably a
consequence of the fact that in the NUV a significant fraction of the
light in spiral galaxies still arises from within the bulge component,
while in the FUV this contribution is in many cases negligible.

The GALEX FUV and NUV observations presented here, along with the
corresponding corollary data in the optical, NIR and FIR provides us
with an unprecedented set of multiwavelength data for a large
population of galaxies in the local Universe. One of the first
questions that can be addressed using this sample concerns the
relation between the qualitative (optical) morphology of these
galaxies and more quantitative properties, such as colors,
luminosities, total-infrared-to-UV ratios, etc. In
Figure 5 we show
the colors of the galaxies as a function of the blue-light
morphological type as given by the RC3.
Panels 5a & 5b show that
although late-type spiral and irregular galaxies are somewhat bluer in
(B - V) and (B - K) than ellipticals and
early-type spirals, these
colors are not unique to a given type. In particular, these colors
cannot be used to unambiguously discriminate between different kinds
of spiral galaxies nor even between elliptical/lenticular galaxies and
spirals. As indicated by
Roberts & Haynes
(1994),
the significant ovelap in (B - V) color between spiral
galaxies of different types is
mostly due to true variations in the optical colors and star-formation
history of galaxies of same morphological type, not to
misclassification or observational errors. The equivalent to the
Panel 5b for late-type Virgo cluster galaxies was obtained by
Boselli et al. (1997).
These authors obtained a large overlap in (B - K)
color between different morphological types as well.

Figure 5. Variation in the observed colors
and total-infrared (TIR) to FUV ratio of the galaxies in the Atlas with
the morphological type (T). a) (B - V) versus the
morphological type for elliptical/lenticular (T < -0.5), spiral
(-0.5 T < 9.5), and
irregular/compact galaxies (T
9.5). The separation
between elliptical/lenticular and the rest is shown by a vertical
dashed line. b) The same for (B - K). Note the
small segregation in color between the different types when the
(B - V) or (B - K) colors are
used. c) The same for (FUV - K). The segregation between
ellipticals/lenticulars and spirals (horizontal solid line) and even
between different kind of spiral galaxies is now remarkable. For
comparison purposes we show (in the same scale) the range in (B
- V) color span by the galaxies in the sample (see panel
a). d) The same for (NUV - K). e) The same
for the (FUV-NUV) color. Note that FUV and NUV magnitudes are in AB
scale and optical and NIR magnitudes are in the Vega system. Green
lines represent the best linear fit to the data for types T = -0.5 or
later (i.e. spiral galaxies). f) The same for the TIR-to-FUV
ratio (see
Buat et al. 2005).

However, thanks to the extreme sensitivity of the FUV data to the
presence of very low levels of recent star formation activity, the use
of the (FUV- K) color turns out to be a very powerful discriminant
between quiescent elliptical and lenticular galaxies, and star-forming
spirals. In particular, an observed (FUV- K) color of 8.8 mag
provides an excellent discrimination point between these two groups
(see Figure 5c). In this sense, of all the
elliptical/lenticular
galaxies in the Atlas with both FUV and K-band data available only
23% of them show a (FUV- K) color bluer than this threshold. It is
worth noting that significant a fraction of these are known to host
some residual star formation activity (e.g. NGC 3265,
Condon, Cotton, &
Broderick 2002
and NGC 0855,
Wiklind, Combes, &
Henkel 1995),
or are low-luminosity ellipticals with obvious star formation
activity like NGC 1510
(Marlowe, Meurer, &
Heckman 1999).
Spiral and irregular galaxies with (FUV- K) colors redder than
this value only represent 9% of the total.

Although with significantly degraded discriminating capabilities
compared to the (FUV- K) color, the (NUV- K) is also well
correlated with the morpholotical type (see
Figure 5d). The same can be said
about the (FUV-NUV) color, where a cut-off at (FUV-NUV) = 0.9 mag
provides a relatively clean separation of elliptical/lenticular
galaxies from spirals (Figure 5e). The fraction of
elliptical/lenticular galaxies with (FUV-NUV) color bluer than
0.9 mag (and both FUV and NUV magnitudes available) is 18% while the
percentage of spiral and irregulars redder than this value is only
12%. Note that in this case the far-left lower corner of the diagram
may be populated both by ellipticals with residual star formation and
also by elliptical galaxies with a strong UV-upturn
(Deharveng, Boselli,
& Donas 2002
and references therein). The best linear fits
derived for the correlation of observed colors with the morphological
type for spirals and irregulars (types T > -0.5) are

(3)(4)(5)

These relations are shown in
Figures 5c, 5d, & 5e. Note that although
the r.m.s. of the fit for the (FUV-NUV) color is smaller than for
(FUV- K) this is purely a consequence of the much smaller dynamic
range of the (FUV-NUV) color (1 mag) compared with the (FUV - K)
color (~ 6 mag) (see Figure 5c). The
corresponding best fits in the type T versus color diagrams are (only
galaxies with types T < 13 are considered)

Finally, in Figure 5f we compare the
total-infrared (TIR hereafter) to
FUV ratio with the morphological type of the galaxies in the
Atlas. The TIR flux was derived using the parameterization of the
TIR-to-FIR ratio given by
Dale et al. (2001),
where FIR is computed from the 60 and 100 micron IRAS fluxes as in
Lonsdale et al. (1985).
The flux in the FUV is expressed in units of
F (see
Buat et al. 2005).
In the case of spiral and irregular
galaxies, for which both the UV and infrared emission are ultimately
due to young massive stars, this ratio provides a well defined
estimator of the dust attenuation in the UV
(Buat et al. 2005;
Cortese et al. 2006).
Given the sensitivity limits of the IRAS
catalog and the low dust content of elliptical and lenticular galaxies
the number of these galaxies detected in both the 60 and 100micron
IRAS bands is only 49 out of the 225 ellipticals in the
Atlas. Figure 5f shows that late-type spirals
and irregulars tend to
show, on average, a lower TIR-to-FUV ratio and consequently smaller
attenuation in the UV than that derived for early-type spirals.

Although morphology is certainly related with the way galaxies form
and evolve, especially when the properties of elliptical and spiral
galaxies are compared, the luminosity and even more the mass (either
the luminous or total mass) is thought to be the main driving force of
the evolution of galaxies through the history of the Universe. In this
sense, the analysis of color-magnitude diagrams (CMD) has
traditionally provided a fundamental tool for understanting galaxy
evolution.

Figures 6a and 6b show
the CMD in (FUV - K) - MK and
(NUV - K) - MK. At the top of these diagrams we find
the `red
sequence' populated primarily by elliptical and lenticular galaxies
(dots). In the case of the (NUV - K) - MK CMD the red
sequence shows
a clear slope with lower luminosity galaxies showing bluer colors,
especially below MK > -23 mag. A similar behavior is seen when
optical or optical-NIR colors are used, both locally and at high
redshift
(Gladders & Yee
2005).
This is commonly explained in terms
of lower metal abundances (thus bluer colors) of the stellar
populations in low mass ellipticals as compared to the more massive
(higher metallicity) systems
(Gladders et al. 1998
and references therein). In the case of the (FUV - K) -
MK CMD, on the other hand,
the distribution of the (FUV - K) color is rather flat over a
range of
almost 7 mag in absolute magnitude. The explanation for this
different behavior can be found in
Figure 6c. Here the (FUV-NUV)
gets systematically redder as we move to lower luminosities. This is
opposite to what is seen in any other colors and it is probably a
consequence of a weaker UV-upturn in intermediate-mass ellipticals
than in the most luminous and massive ones (see
Boselli et al. 2005).
Note that, due to the stronger UV-upturn towards the centers of
elliptical galaxies
(Ohl et al. 1998;
Rhee et al. 2006,
in preparation), the asymptotic colors do not probably show the full
strength of the UV-upturn in the way aperture colors like those
obtained from the analysis of IUE spectra do
(Burstein et al. 1988).

Figure 6. Color-magnitude diagrams (CMD) of
the Atlas galaxies.
Red dots are elliptical/lenticular galaxies, dark green triangles are
early-type spirals (T < 5), light green triangles are late-type
spirals (T 5), blue
asterisks are irregular and compact galaxies, and black diamonds are
galaxies currently lacking morphological classification. a)
(FUV - K) - MK CMD. Spiral and irregular galaxies show
a systematic bluing as we move to galaxies of lower
mass. Elliptical/lenticular galaxies, on
the other hand, show a very small change in the (FUV - K) color with
the K-band absolute magnitude (i.e. stellar mass) of the
galaxy. b) (NUV - K) - MK CMD. In this case,
however, lower
mass ellipticals are also systematic bluer than more massive
ones. c) (FUV-NUV)-MK CMD. This plot shows that the
behavior observed in the elliptical galaxies in previous diagrams
seems to be consequence of a much fainter UV upturn (best traced by
the FUV-NUV color) in low-luminosity ellipticals than in massive
ones. In this plot we show the position occupied by dwarf elliptical
galaxies in Virgo
(Boselli et al. 2005).
Dwarf elliptical galaxies
fainter than MK < -21 mag start to show the effects of recent
star formation both on their (FUV-NUV) and UV-optical colors (see
Boselli et al. 2005 for more details). d)
(FUV-NUV)0-MK CMD. The (FUV-NUV)0 color
is corrected for
internal extinction using the relation between the total-infrared
(TIR) to FUV ratio and the extinction in the FUV and NUV bands given
by Buat et al. (2005).
Only spiral and irregular/compact galaxies
are used in this plot. Solid (dashed) line represents the best
weighted (non-weighted) fit to the data. The narrow distribution in
extinction-corrected UV slopes indicates that the tendency seen in the
(FUV-NUV)-MK CMD shown above for spiral galaxies is a direct
consequence of the change in the amount of dust with the luminosity of
the galaxy. e) (FUV-NUV)-MFUV CMD. f)
(FUV-NUV)-MB CMD. These two latter diagrams show a similar
behavior to that shown in panel c.The high-luminosity end of the
sample in the FUV is clearly dominated by spiral galaxies with a very
narrow distribution in observed (FUV-NUV) color.

Dwarf elliptical galaxies have K-band absolute magnitudes that are
typically fainter than MK = -21 mag. Unfortunately, not many of
these more extreme low-luminosity ellipticals are found in the
Atlas. This is mainly because dwarf ellipticals in Virgo (where most
of the studies on dE have been carried out to date) are typically
smaller than 1arcmin in size placing them outside the selection
limit imposed on the Atlas. Nevertheless, a recent study by
Boselli et al. (2005)
suggests that residual star formation might play a
leading role in the interpretation of the UV emission from dE
galaxies, which would explain their behavior in the CMD (i.e.
similar to the behavior seen in low mass star-forming galaxies). The
tendency for the most luminous ellipticals to show bluer (FUV-NUV)
colors is even more clear when the FUV-band absolute magnitude is
considered (see Figure 6e). However, if the
B-band luminosity is
used, the (FUV-NUV) color seems to be independent of luminosity.

Regarding the properties of spiral (triangles) and irregular galaxies
(asterisks) in these plots we find that the majority of these galaxies
are concentrated in a `blue sequence' with high-luminosity spirals
(which also tend to be of earlier types) being redder than low-mass
spirals and irregular/compact galaxies. This is true for all the
observed (FUV - K), (NUV - K), and (FUV-NUV) colors
(Figures 6a, 6b, & 6c).
There are two mechanisms that may lead to the observed
behavior. First, low luminosity galaxies are known to have lower
metallicities (both in the stars and in the gas) than more luminous ones
(Salzer et al. 2005
and references therein). This implies that
the amount of dust (and reddening of the colors) in low-luminosity
galaxies should be lower than in luminous ones.

The (FUV-K) [(NUV-K)] color is found to span a range of 5 mag
[4 mag] in spiral and irregular galaxies of different types and
luminosities with a mean value of 5.9 mag [5.4 mag]. The
corresponding 1-sigma of the distribution is 1.7 mag [1.4 mag]. On
the other hand, the dispersion in the AFUV
[ANUV] derived is only 1.0 mag [0.8 mag] (see
below). Since the AFUV / (AFUV-AK)
[ANUV / (ANUV-AK)] total-to-selective
extinction ratio is always between 1.0 and 1.1 for any attenuation law
considered, dust extinction alone is not able to explain the
dispersion in the observed (FUV-K) [(NUV-K)] color neither its
dependence on luminosity or morphological type.

Since we have information about the TIR emission for a large fraction
of these galaxies we can compute the attenuation in the FUV and NUV
from the observed TIR-to-FUV ratio using the recipes published by
Buat et al. (2005).
The mean and 1-sigma FUV [NUV] attenuation of the
sample of spiral and irregular galaxies in the Atlas is
1.8 ± 1.0 mag [1.3 ± 0.8 mag]. The extinction-corrected
(FUV-NUV) color is plotted in Figure 6d as a
function of the K-band absolute magnitude. The solid (dashed)
line shown in this plot represents the best weighted (non-weighted) fit
to the data

(9) (10)

Although there is a small tendency for the galaxies to show redder UV
colors at lower luminosities and later types, we do not exclude the
possibility that the intrinsic (FUV-NUV) color derived in this way
is independent of luminosity with an average value of
(FUV-NUV)0 = 0.025 ± 0.049 mag (i.e.
GLX,0
= -1.94 ± 0.11; see
Kong et al. 2004).
We should note here that the
measurements of the extinction in the FUV and NUV from which this
intrinsic (FUV-NUV) color is derived are not fully independent since
both are obtained by comparing the corresponding observed FUV and NUV
flux with the same total-infrared emission
(Buat et al. 2005).
Consequently, there might be some additional weak dependency of
the intrinsic (FUV-NUV) color with the luminosity that could be
identified by analyzing both the detailed star formation history and
dust properties (composition, geometry, temperature distribution) of
individual galaxies.

Figure 6e shows that the most luminous galaxies
in the FUV are spirals (both early- and late-type ones). In the optical
(Figure 6f) and NIR
(Figure 6c), on the other hand, the bright end
of the luminosity
function is populated by both elliptical and spiral galaxies. It is
also worth noting that the galaxies in the bright end of the FUV
luminosity function show a very narrow dispersion in the observed
(FUV-NUV) color, that results in a very similar shape for the bright
end of the FUV and the NUV local luminosity functions
(Wyder et al. 2005).

In Figure 7a we analyze the (FUV-NUV)-(NUV-
B) color-color diagram
of the galaxies in the Atlas. It is remarkable the relatively narrow
strip of this diagram where the galaxies are located. In the case of
the spiral galaxies this is due in part to the well-known degeneracy
in these colors between dust extinction and star formation history
(see e.g.
Gil de Paz & Madore
2002).
The ellipticals show a very
narrow range in (NUV - B) color but a wide range of (FUV-NUV)
colors, probably due to differences in the strength of the UV-upturn
from galaxy to galaxy. In the (FUV-NUV) - (NUV - K) color diagram
(Figure 7b) we find that ellipticals with redder
(NUV - K) color tend
to show bluer (FUV-NUV) colors. This is again a consequence of the
weaker UV-upturn present in optically blue, intermediate-mass
ellipticals. The combination of the (FUV-NUV) color with either the
(NUV - B) or the (NUV - K) color clearly improves the
discrimination
between elliptical/lenticular galaxies and spirals (see broken lines
in Figures 7a & 7b). In the case of the
(FUV-NUV) -(NUV - B)
color-color diagram the origin {destination} of the cut-off line is
[(FUV-NUV),(NUV - B)] = [2.0,2.0] {5.0,0.0}. For the
(FUV-NUV)-(NUV - K) color-color diagram the corresponding origin
{destination} of the cut-off line is [(FUV-NUV),(NUV - K)] =
[5.0,1.7] {9.5,0.4}.

Figure 7. Color-color diagrams of the
galaxies in the Atlas. a) (FUV-NUV)-(NUV- B) color-color
diagram. b) (FUV-NUV)-(NUV- K) color-color
diagram. Symbols have the same meaning as in Figure 6. Lines in this
plot represent various criteria proposed to separate
elliptical/lenticular galaxies from spirals (see text for
details). Note that, in order to keep with the stellar convention, the
(FUV-NUV) axis has been flipped and red (FUV-NUV) colors are now
plotted at the bottom of the figure.

The relation found by
Heckman et
al. (1995) and
Meurer et al. (1995,
1999)
between the TIR-to-FUV ratio and the slope of the UV
spectrum in starburst galaxies
(IRX-
relationship; see also
Seibert et al. 2005)
can be used in principle to estimate the dust
extinction in galaxies even if FIR data are not available. Some recent
works have claimed that this relationship is valid only when applied
to UV-selected starburst galaxies but not in the case of
infrared-bright objects like the luminous/ultra-luminous infrared
galaxies (LIRGs/ULIRGs;
Goldader et al. 2002)
or even for normal spiral or irregular galaxies (see
Bell et al. 2002
for results on the LMC). In Figure 8a we compare
the TIR-to-FUV ratio with the observed (FUV-NUV) color, which is
equivalent to the slope of the UV continuum (see
Kong et al. 2004).
Here we have only plotted galaxies
with observed (FUV-NUV) color bluer than 0.9 mag. This criterion
guarantees that the vast majority of the objects considered are either
spiral or irregular galaxies. The dotted line represents the
IRX-
relation given by
Meurer et al. (1999).
This figure demonstrates that the slope of the UV is indeed well
correlated with
the TIR-to-FUV and can be used to estimate (at least in a statistical
way) the dust extinction in nearby galaxies. Similar results are found
by Cortese et
al. (2006)
using a volume-limited optically-selected
sample of galaxies in nearby clusters.

Figure 8.a) IRX-beta
relation. The
vertical long dashed-line represents the cutoff in (FUV-NUV) color used
to select the sub-sample of galaxies used to study the relation between
the TIR-to-FUV ratio and the slope of the UV. This selection criterion
guarantees that in the galaxies considered both the infrared and the UV
emission are in the most part associated with the presence of recent
star formation activity. The dotted line represents the relation
derived by using a sample of starburst galaxies
(Kong et al. 2004;
Meurer et al. 1999).
The best fit to the whole set of data is shown by a solid line. The
best fit obtained excluding the lowest luminosity galaxies (dots) is
shown by a dashed line. Symbols are coded by UV luminosity. Galaxies
with higher UV luminosities (green stars) seem to be located somewhat
closer to the relation derived for starburst galaxies that fainter
objects (blue squares). Triangles correspond to the elliptical galaxies
in the sample. Note that most of the ellipticals with (FUV-NUV) <
0.9 are known to have some degree of residual star formation. b)
AFUV versus (FUV-NUV)-0.025. The latter term is equivalent
to AFUV-ANUV if an intrinsic (FUV-NUV) = 0.025
mag is assumed for all star-forming galaxies in the sample (see
Section 5.3). The lines drawn correspond to
the total-to-selective extinction in the UV expected for different
extinction laws (MW, solid line; LMC 30 Doradus, dotted line; SMC Wing,
dashed line; SMC Bar, dot-dot-dot-dashed line) and the attenuation law of
Calzetti et al.
(1994, dot-dashed
line).
The RV values adopted for each of these laws are
given in the text. Note that the inclusion of scattering would result
in steeper relations between AFUV and
AFUV-ANUV than those shown here. Therefore, an
attenuation law based on the SMC Bar extinction law seems to be favored
by these results. High-UV-luminosity galaxies are still adequately
represented by the Calzetti attenuation law.

The solid line in Figure 8a represents the best
linear fit to the
data. The dashed line is the same but excluding objects with
luminosities below 0.1 × L*
(M*FUV = -18.12;
Wyder et al. 2005),
for which the relation begins to depart from
linearity. The results of these fits are

(11) (12)

Note that our sample suffers of a small deficiency of low-luminosity
spirals. This fact might have an impact on the best-fit
IRX-
relationship derived above.
Cortese et al. (2006)
have recently
proposed a set of recipes that can be used to estimate the TIR-to-FUV
ratio in star-forming galaxies using not only the (FUV-NUV) color
but other parameters such as the oxygen abundance, the luminosity, the
mean surface brightness, etc.

The majority of the objects in Figure 8a are
found below the
relationship defined for starburst galaxies. It is worth noting that
objects with higher UV luminosity, some of them starburst galaxies,
seem to fall closer on average [at least in the region with
(FUV-NUV) < 0.6 mag] to Meurer et al.'s relation than lower
luminosity galaxies. According to
Kong et al. (2004)
the offset
between normal galaxies and starbursts is primarily due to a lower
ratio of present to past-averaged SFR in normal galaxies. However, the
results obtained by
Seibert et
al. (2005) and
Cortese et al. (2006)
using GALEX data of nearby galaxies do not support this
idea. These recent studies suggest that this offset might be due
instead to a different geometry of the dust in normal galaxies
compared with starbursts or, alternatively, to aperture effects
present in the IUE dataset used by
Meurer et al. (1999).

The fact that we find such a good correlation between the TIR-to-FUV
ratio and the (FUV-NUV) color and that the intrinsic (FUV-NUV)
color seems to be rather constant for spiral and irregular galaxies
suggests that the attenuation law in the UV for these galaxies is
different from a pure Galactic extinction law. In the case of the
Milky Way the extinction law shows a bump at 2175Å that would
result in a similar extinction in both bands,
AFUV = 7.9 × E(B - V) and
ANUV = 8.0 × E(B - V)
(Bianchi et al. 2005).
Thus, the observed trend in the (FUV-NUV) color with the TIR-to-FUV ratio
is most probably due to a different extinction law since scattering,
either for a shell or clumpy dust geometry, would result in an even
lower FUV attenuation (compared with the NUV) than that expected from
the Galactic extinction law alone (see e.g.
Roussel et al. 2005).
The SMC Bar or 30 Doradus extinction laws and the attenuation
law proposed by
Calzetti et al. (1994)
all show a weak 2175Å feature and, especially in the case of the
SMC Bar extinction law, a
relatively steep FUV rise. In this sense, despite of including
scattering, the FUV rise of the Calzetti law is apparently too modest
to reproduce the dependence between AFUV and
AFUV-ANUV followed by the majority of
the galaxies in our sample (see Figure 8b)
. Thus, although the Calzetti
law, originally
built for UV-bright starburst galaxies, still provides an adequate
approximation to the relation between AFUV and
AFUV-ANUV for galaxies with UV
luminosities above L*, an attenuation law
based on the SMC-Bar
extinction law is favored for the bulk of the galaxies in this Atlas.

We cannot exclude, however, that the FUV emission might be arising
from young stars more deeply embedded in their parent molecular clouds
than those responsible for the NUV emission. If that is the case, the
differential extinction between the FUV and NUV emitting sources would
lead to an artificial FUV rise in the global attenuation law even if
the extinction law is rather flat in the UV.

In this same sense, it is worth noting that here we are referring to
the attenuation law of the dust associated with the regions
responsible for the UV emission, which could be quite different from
the law we would obtain from regions dominating the emission at other
wavelengths and also different from the extinction law that would be
derived from line-of-sight absorption studies of individual stars.

Concentration indices have been commonly used in the past to infer the
morphological types of barely resolved intermediate redshift galaxies
found in HST images (see e.g.
Abraham et al. 1996).
One of the
problems associated with these studies is the fact that in many cases
the concentration indices derived for the high-redshift galaxies are
measured in the rest-frame UV while the local reference samples are
usually observed in the optical
(Bershady, Jangren,
& Conselice 2000).
In this sense, it is important to know the structural
parameters in the UV of a sample of well-known nearby galaxies, like
the one collected for this Atlas. Concentration indices C31 and C42
are provided in Table 3. The number
of objects with these indices is
small because we only computed the C31 (C42) concentration index for
those galaxies whose radius containing 25% (20%) of the light was
larger than 6arcsec. The same criterion applies to the effective
radius, where the radius containing 50% of the light was imposed to
be larger than 6arcsec in order for it to be measured.

In Figure 9 we compare the concentration index C42
with the (FUV- K)
color. As we commented in Section 5.2 this
color discriminates very well between elliptical and spiral galaxies
and also between spiral galaxies of different types (see
Section 5.2). This figure shows that the C42
index improves the discrimination between ellipticals (dots) and
lenticulars (open circles) and also between these and early-type
spirals (open triangles).
Joe et al. (2006, in preparation)
have recently carried out a more detail study of the structural properties
(including both concentration and asymmetry parameters) of nearby
galaxies in the UV using the same sample presented in this Atlas.

Figure 9. (FUV- K) color versus the
concentration index C42 in the NUV. Symbols are coded by morphological
type. Although the galaxies are better segregated in (FUV - K)
color than in concentration index, the value of C42 can be used improve
to the discrimination between ellipticals and lenticulars and between
these and some early-type spirals.

Regarding the morphological classification of the UV
surface-brightness profiles we first notice a large variety of
morphologies even within each of the classes defined in
Section 4.4. This is partly a
consequence of the
high sensitivity of the UV to the recent star formation which results
in the presence of structures having relatively short evolutionary
time-scales that might dominate the UV profiles but that are not as
obvious in the optical or NIR profiles. There is also the difficulty
of dealing with degeneracies between some morphological classes. In
this sense, some of the Blue Compact Dwarf galaxies in the sample
could be easily classified as having ER or EV profiles. Also, some of
the profiles inspected could be either classified as EEh or
Ed. Despite of these issues we successfully classify the profiles of
970 of the 1034 galaxy in the Atlas. Moreover, we find that most of
the galaxies (615 out of 970) have UV profiles that can be grouped in
three main classes: (1) profiles that can be reproduced entirely by a
de Vaucouleurs law (class VV), (2) pure exponential profiles (class
EE), (3) profiles with an exponential component in the outer region
and significant flattening in the inner region (EF and Ef
classes). Only 19 galaxies were classified as EV class, despite being
the dominant morphology in the optical and near-infrared profiles of
spiral galaxies.

This paucity of EV profiles seems to be due, at least in the case of
late type spirals, to the fact that even in the central regions the
bulge is much fainter than the disk, which results in these galaxias
being classified as having type EE or EF/Ef profiles (e.g. NGC 0628,
M 33, NGC 1042, NGC 2403). In early-type spirals, like the Sb galaxies
NGC 0986, M 31, M 81, M 95, the bulge is dominant only in the nucleus
of the galaxy where is also commonly found associated with a
flattening or decrease in the surface brightness of the disk toward
the center. Because of the small spatial extension of these bulges in
the UV surface brightness profiles Sb galaxies get usually classified
as EFn, VFn, or EDn. Only lenticulars (e.g. NGC 1387, NGC 1546,
NGC 4310, NGC 4477, NGC 6945, NGC 7252, M 86), intermediate S0/a
(NGC 2681, NGC 3816, NGC 3885, IC 0796), or very early-type spirals
like the Sa galaxies NGC 1022, NGC 2798, NGC 4314, or NGC 4491, are
sometimes best classified as having EV-type UV profiles. This is true
for both UV bands although it is more frequent in the case of the NUV
profile.

In Figure 10 we plot the distribution of
galaxies classified within
each of these groups: de Vaucouleurs profiles (v), pure
exponential profiles (e), and flattened exponential profiles
(f); in the (FUV - K) versus morphological type
diagram. Again,
the morphological types used are those published in the RC3. In the
light of this figure it is fair to say that the majority of the
elliptical galaxies in the Atlas follow a de Vaucouleurs profile in
the UV, like is the case of the optical and NIR profiles of luminous
elliptical galaxies. Note that because of our selection limits a small
number of dwarf elliptical galaxies (which commonly show exponential
light profiles in the optical) is expected to be found in this
Atlas. A few v-type galaxies classified morphologically as late
T-type objects are found to be well-known Blue Compact Dwarf (BCD)
galaxies: NGC 1569, NGC 3125, NGC 5253, NGC 6789, UGC 05720 (Haro 2). See Doublier et al.
(1997,
1999)
for some other examples of BCD
galaxies with R1/4 profiles in the optical.

Figure 10.a) (FUV- K) color
versus the morphological type. The symbols are coded by letters that
represent the morphology of their UV profiles: v for galaxies
following a de Vaucouleurs R1/4 profile, e for
galaxies with pure exponential profiles, and f for galaxies with
exponential profiles in the outer regions and a flattened profile
inside. b) Morphological-type distribution for each class of UV
profile.

Regarding the distribution of the other two types of profiles we point
out that while galaxies with pure exponential profiles (a total of
173) are widely distributed in morphological type and color, galaxies
with flattened exponential profiles (269) have, in the majority of the
cases, morphological types T in the range 2 < T < 8, i.e. they are
truly spiral galaxies. In order to explain this behavior is necessary
to understand first what is the mechanism(s) behind the flattening of
the UV profiles.

In the spectro-photometric models of the evolution of disk galaxies of
Boissier & Prantzos
(2000;
see also Boissier 2000),
a similar
flattening in blue bands is obtained. The main reason for it is that
the rate the stars formed (i.e. SFR) in the inner disk has been
higher than the infall of gas, leading to a progressive consumption of
the gas in these regions. In the outer parts, however, star formation
is less efficient and infall proceeds on longer timescales. As a
result, the gas reservoir of the outer disk is not exhausted, and the
shape of the exponential profile is preserved (in adition, an
extinction gradient could enhance the difference between the inner
regions, metal and dust rich, and outer regions suffering low
metallicity and low extinction).

The dependence of the degree of flattening with the morphological type
found, with most galaxies showing flattened-exponential profiles
having types Sab-Sdm, is probably a consequence of the fact that (1)
early-type galaxies have already consumed the majority of their gas at
all radii, due to a high global star formation efficiency and low
current infall, and (2) late-type spirals, because of their current
large supply of gas and infall, still have enough gas to prevent its
consumption at all radii. Note also that in some very early type
spirals (S0/a and Sa types) the presence or a relatively bright bulge
might also difficult the detection of any flattening in the inner-disk
profile.

The models referred above use as parameters the circular velocity
(i.e. total mass), and the spin parameter (i.e. angular
momentum). For a fixed spin parameter, the degree of flatenning should
depend on mass since e.g. the infall time-scale depends on the
mass. Indeed, at very low mass a modest flattening occurs, a more
visible one at intermediate mass, and no flatenning again in very
massive galaxies (where the gas has been consumed over the whole
galaxy)
(Boissier 2000).
However, using the K-band absolute magnitude
as a tracer of the total mass of the system we found no difference
between the distribution of galaxies with or without flattening in
their profiles. This disagreement with the naive expectation from the
models could be linked to the existence of the second parameter (at
fixed velocity, the flattening of the star formation rate is more
noticeable for smaller spin parameters), or more fundamental
differences between EF/Ef and EE galaxies, not yet included in models.
A more direct measure of the total mass and spin parameter, or
detailed modeling of these galaxies (or a sub-sample of them) could
help us to understand what makes the EF/Ef galaxies different from the
EE ones.